Plasma processing is commonly used in processing of semiconductors and other products. Plasmas can be used for a variety of processing operations including deposition, etching, oxidation, removal of organic material, and other treatments. Generating a plasma involves exposing an input gas to energy input in the form of high electric and/or magnetic fields. At least some of the molecules of the input gas are excited by the fields and gain energy, in some cases becoming ionized.
Processing involves exposing the product to ions or reactive species generated in the plasma, and/or gas molecules that were present in the plasma but were not excited (any combination of plasma-generated ions, reactive species, and/or gas molecules that were not excited by the plasma will be referred to as “plasma products” herein). Generally speaking, there are two modes of plasma processing. In in-situ processing, item(s) being processed are in the location where the plasma is generated. In remote plasma processing, a plasma is generated in a first location, and the plasma products are brought to a second location, where they contact the item(s) being processed. Gas flows, vacuum pumping, electric fields and/or magnetic fields may be utilized to direct the plasma products to the product being processed. Remote treatment is sometimes preferable because parameters that affect the plasma can be controlled at the first location, but at the second location containing the product, the high electric or magnetic fields used to generate the plasma, and/or high velocity ions produced in the plasma, can be reduced or eliminated to avoid damage to certain types of product. Some plasma processing systems provide both in-situ and remote plasma processing capabilities.
Two known methods of generating a plasma are capacitive coupling and inductive coupling. In a capacitively coupled plasma, a high frequency (usually radio frequency, or RF) electric field is applied directly to the input gas to generate the plasma. In an inductively coupled plasma, a magnetic field is provided within a chamber containing the input gas. The magnetic field is often generated by a high power RF signal being transmitted into a coil, so that the magnetic field is generated within the coil, and generates transverse electrical currents within the gas according to the right-hand rule. Some capacitive coupling is often utilized to initiate the plasma by inducing a Townsend avalanche in the input gas, generating charge carriers for the transverse electrical currents. In some cases the coil is wrapped around the location where the plasma is generated; in others coil is wrapped around a magnetic (e.g., ferrite) core to enhance and/or direct the magnetic field to another location where the plasma is generated.